Ignore not existing parts.

pkg/analyzer/test/src/task/dart_test.dart

Index: pkg/analyzer/test/src/task/dart_test.dart
diff --git a/pkg/analyzer/test/src/task/dart_test.dart b/pkg/analyzer/test/src/task/dart_test.dart
index e70d9f769391cbe16661799781fcb71077ce31ce..130cf16e58a957020460dd01bd9741ec184d549d 100644
--- a/pkg/analyzer/test/src/task/dart_test.dart
+++ b/pkg/analyzer/test/src/task/dart_test.dart
@@ -899,6 +899,17 @@ part of libB;
     expect(error.getProperty(ErrorProperty.PARTS_LIBRARY_NAME), isNull);
   }
 
+  test_perform_error_partDoesNotExist() {
+    _performBuildTask({
+      '/lib.dart': '''
+library lib;
+part 'part.dart';
+'''
+    });
+    // we already report URI_DOES_NOT_EXIST, no need to report other errors
+    _assertErrorsWithCodes([]);
+  }
+
   test_perform_error_partOfDifferentLibrary() {
     _performBuildTask({
       '/lib.dart': '''